This invention relates to epoxy resin compositions. In one aspect, the invention relates to high-modulus epoxy compositions suitable for use in advanced composites.
It is known that aromatic diamines and polyamines are useful curing agents for epoxy resin compositions in which high heat resistance, good elongation and relatively long pot life are desired. These properties are often retained or enhanced in the presence of small amounts of a viscosity-reducing styrenic monomer diluent and a crosslinking codiluent such as trimethylolpropane timethacrylate. However, in many cases, aromatic amine-cured epoxies have lower modulus than would be desired, especially for use as the matrix phase in advanced composites. This is particularly true of glycidyl ethers of bisphenols (such as the diglycidyl ether of bisphenol-A) cured with orthoalkylated aromatic diamines such as diethyltoluenediamine (DETDA). Such orthoalkylated aromatic diamines have become popular because of claims of reduced toxicity in comparison with non-orthoalkylated aromatic diamines or polyamines, such as methylenedianiline (MDA). However, the problem of low modulus can become severe in diepoxy resins cured with some of the orthoalkylated aromatic diamines, especially DETDA.
A number of methods exist in the art for raising the modulus of a cured epoxy resin. One of these is to add a multfunctional resin (such as N,N,N',N'-tetraglycidyl methylenedianiline, or a glycidated phenol or o-cresol novolac) to the system to replace part or all of the diglycidyl bisphenol. This method can raise the modulus very effectively. However, the increase in crosslink density produced by the multifunctional resin often lowers the tensile elongation of the cured epoxy to 2% or less. Another method of increasing the cured epoxy modulus involves adding antiplasticizers or "fortifiers" such as the adduct of phenyl glycidyl ether with p-hydroxyacetanilide. These materials are typically unreactive but may contain some reactive epoxy groups. The "fortifiers" are claimed to increase cured epoxy modulus by filling much of the free volume in the cured epoxy network. However, the use of these antiplasticizers or "fortifiers" has a number of disadvantages. For example, the fortifiers, especially the unreactive ones, tend to reduce both the heat resistance and the break elongation of the cured epoxy. These effects become more pronounced as the fortifier concentration is increased to raise the modulus further. The reactive fortifiers, which contain some epoxy or epoxy-reactive groups, cause less loss in heat resistance and tensile elongation. However, the reactive fortifiers are also less effective than the unreactive ones in raising the modulus.
Other methods of raising the modulus of an amine-cured epoxy resin include undercuring the resin and the use of an excess of resin with respect to curing agent. The first is undesirable because the heat resistance is lowered with respect to the fully cured network. The second is associated with both lowered heat resistance and lowered tensile elongation. Simple alicyclic anhydrides, such as methyl tetrahydrophthalic anhydride, yield cured epoxy resins which are much higher in modulus than typical aromatic amine-cured epoxies and also have good tensile elongation (in the 6-8% range). However, these anhydrides yield cured networks which are considerably lower in T.sub.g or HDT than standard aromatic-amine cured epoxies. The anhydride-cured networks are also held together by hydrolytically sensitive ester linkages.
It is therefore an object of the invention to increase the modulus of an aromatic amine-cured epoxy network. In one aspect, it is an object of the invention to prepare an epoxy resin composition which, in the cured state, has a combination of high modulus, good heat resistance and high tensile elongation.